[llvm] [TableGen] Support for optional chain in Selection DAG nodes (PR #163079)

[Serge Pavlov via llvm-commits]

spavloff wrote:

The possible solution to use mandatory chain argument is discussed in https://discourse.llvm.org/t/rfc-change-of-strict-fp-operation-representation-in-ir/85021. 

https://github.com/llvm/llvm-project/pull/163079